This invention relates to a superconducting coil protective system and a protective method for protecting a superconducting coil from being destroyed upon quenching.
FIGS. 1 to 4 illustrate circuit diagrams of various superconducting coil protective systems disclosed in "Improvements in the Parallel Resistor Circuit for the Quench Protection of a Superconducting Magnet" by T. Nakano, S. Okuma and Y. Amamiya, B-102 No. 12 pp. 73-79, published by the Japanese Institute of Electrical Engineering in, December, 1982.
In the conventional parallel resistor circuit type protective system illustrated in FIG. 1, a cryostat CR comprises a superconducting coil L and a resistance R(t) of a normal conduction portion generated in the superconducting coil L. This resistance R(t) increases as time passes. Across the cryostat CR, a power source such as a mono-polar electric source E is connected through a power switch S, and a protective resistor R.sub.D is connected in parallel to the cryostat CR.
FIG. 2 illustrates a protective circuit in which a diode D is employed in place of the protective resistor R.sub.D and two switches S1 and S2 as well as three resistors R1, R2 and R3 are used to form a multi-stage parallel resistance.
FIG. 3 illustrates a protective circuit in which series-connected resistors Ra and Rb are connected in parallel to the cryostat CR, and a capacitor C is connected across the resistor Rb. The protective circuit illustrated in FIG. 4 further comprises a series circuit of an inductor Ls and a resistor Rs connected in parallel to the protective resistor.
All of these known protective circuits illustrated in FIGS. 1 to 4 comprise the power switch S connected between the power source E and the superconducting coil L. This power switch S is closed during normal operation, and a very large current from the power source E flows through the superconducting coil L, but substantially no current flows through the protective resistor R.sub.D because it has a large resistance.
However, upon the occurrence of quenching in the superconducting coil L, in order to quickly remove stored energy within the superconducting coil L, as soon as the occurrence of the quenching in the superconducting coil L is detected, the voltage of the power source E is decreased and at the same time the power switch S is opened. Then, a high voltage Vc which generated across the power switch S is applied to the protective resistor R.sub.D, whereupon an electric current which previously flowed through the superconducting coil L begins to flow as indicated by the arrow i.sub.c. Then, the magnetic energy stored in the superconducting coil L is converted into heat in the protective resistor R.sub.D to be dissipated to the exterior of the cryostat CR, whereby the superconducting coil L can be protected.
With the conventional protective system discussed above, the power switch must carry an extremely massive current, which also flows through the superconducting coil, during normal operating condition, and the power switch must interrupt this massive current at a high voltage upon the occurrence of quenching in the superconducting coil. In fact, the design specification for a superconducting plasma experiment apparatus (also called LHD) planned by the Fusion Science Laboratory of the, Ministry of Education of the, Japanese Government includes a continuous current of 20 kA, to 30 kA and a voltage of 6 kV is generated at the time of interruption. A power switch having ratings which satisfy the above design specification is inevitably very large and very expensive.